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<div class="section">
<div class="titlepage"><div><div>
<h2 class="title">
<a id="_best_fields"></a>最佳字段<a class="edit_me edit_me_private" rel="nofollow" title="Editing on GitHub is available to Elastic" href="https://github.com/elasticsearch-cn/elasticsearch-definitive-guide/edit/cn/110_Multi_Field_Search/15_Best_field.asciidoc">edit</a>
</h2>
</div></div></div>
<p>假设有个网站允许用户搜索博客的内容，以下面两篇博客内容文档为例：</p>
<div class="pre_wrapper lang-sense">
<pre class="programlisting prettyprint lang-sense">PUT /my_index/my_type/1
{
    "title": "Quick brown rabbits",
    "body":  "Brown rabbits are commonly seen."
}

PUT /my_index/my_type/2
{
    "title": "Keeping pets healthy",
    "body":  "My quick brown fox eats rabbits on a regular basis."
}</pre>
</div>
<div class="sense_widget" data-snippet="snippets/110_Multi_Field_Search/15_Best_fields.json"></div>
<p>用户输入词组 “Brown fox” 然后点击搜索按钮。事先，我们并不知道用户的搜索项是会在 <code class="literal">title</code> 还是在 <code class="literal">body</code> 字段中被找到，但是，用户很有可能是想搜索相关的词组。用肉眼判断，文档 2 的匹配度更高，因为它同时包括要查找的两个词：</p>
<p>现在运行以下 <code class="literal">bool</code> 查询：</p>
<div class="pre_wrapper lang-sense">
<pre class="programlisting prettyprint lang-sense">{
    "query": {
        "bool": {
            "should": [
                { "match": { "title": "Brown fox" }},
                { "match": { "body":  "Brown fox" }}
            ]
        }
    }
}</pre>
</div>
<div class="sense_widget" data-snippet="snippets/110_Multi_Field_Search/15_Best_fields.json"></div>
<p>但是我们发现查询的结果是文档 1 的评分更高：</p>
<div class="pre_wrapper lang-js">
<pre class="programlisting prettyprint lang-js">{
  "hits": [
     {
        "_id":      "1",
        "_score":   0.14809652,
        "_source": {
           "title": "Quick brown rabbits",
           "body":  "Brown rabbits are commonly seen."
        }
     },
     {
        "_id":      "2",
        "_score":   0.09256032,
        "_source": {
           "title": "Keeping pets healthy",
           "body":  "My quick brown fox eats rabbits on a regular basis."
        }
     }
  ]
}</pre>
</div>
<p>为了理解导致这样的原因，需要回想一下 <code class="literal">bool</code> 是如何计算评分的：</p>
<div class="olist orderedlist">
<ol class="orderedlist">
<li class="listitem">
它会执行 <code class="literal">should</code> 语句中的两个查询。
</li>
<li class="listitem">
加和两个查询的评分。
</li>
<li class="listitem">
乘以匹配语句的总数。
</li>
<li class="listitem">
除以所有语句总数（这里为：2）。
</li>
</ol>
</div>
<p>文档 1 的两个字段都包含 <code class="literal">brown</code> 这个词，所以两个 <code class="literal">match</code> 语句都能成功匹配并且有一个评分。文档 2 的 <code class="literal">body</code> 字段同时包含 <code class="literal">brown</code> 和 <code class="literal">fox</code> 这两个词，但 <code class="literal">title</code> 字段没有包含任何词。这样， <code class="literal">body</code> 查询结果中的高分，加上 <code class="literal">title</code> 查询中的 0 分，然后乘以二分之一，就得到比文档 1 更低的整体评分。</p>
<p>在本例中， <code class="literal">title</code> 和 <code class="literal">body</code> 字段是相互竞争的关系，所以就需要找到单个 <em>最佳匹配</em> 的字段。</p>
<p>如果不是简单将每个字段的评分结果加在一起，而是将 <em>最佳匹配</em> 字段的评分作为查询的整体评分，结果会怎样？这样返回的结果可能是： <em>同时</em> 包含 <code class="literal">brown</code> 和 <code class="literal">fox</code> 的单个字段比反复出现相同词语的多个不同字段有更高的相关度。</p>
<div class="section">
<div class="titlepage"><div><div>
<h3 class="title">
<a id="dis-max-query"></a>dis_max 查询<a class="edit_me edit_me_private" rel="nofollow" title="Editing on GitHub is available to Elastic" href="https://github.com/elasticsearch-cn/elasticsearch-definitive-guide/edit/cn/110_Multi_Field_Search/15_Best_field.asciidoc">edit</a>
</h3>
</div></div></div>
<p>不使用 <code class="literal">bool</code> 查询，可以使用 <code class="literal">dis_max</code> 即分离 <em>最大化查询（Disjunction Max Query）</em> 。分离（Disjunction）的意思是 <em>或（or）</em> ，这与可以把结合（conjunction）理解成 <em>与（and）</em> 相对应。分离最大化查询（Disjunction Max Query）指的是： <em>将任何与任一查询匹配的文档作为结果返回，但只将最佳匹配的评分作为查询的评分结果返回</em> ：</p>
<div class="pre_wrapper lang-sense">
<pre class="programlisting prettyprint lang-sense">{
    "query": {
        "dis_max": {
            "queries": [
                { "match": { "title": "Brown fox" }},
                { "match": { "body":  "Brown fox" }}
            ]
        }
    }
}</pre>
</div>
<div class="sense_widget" data-snippet="snippets/110_Multi_Field_Search/15_Best_fields.json"></div>
<p>得到我们想要的结果为：</p>
<div class="pre_wrapper lang-js">
<pre class="programlisting prettyprint lang-js">{
  "hits": [
     {
        "_id":      "2",
        "_score":   0.21509302,
        "_source": {
           "title": "Keeping pets healthy",
           "body":  "My quick brown fox eats rabbits on a regular basis."
        }
     },
     {
        "_id":      "1",
        "_score":   0.12713557,
        "_source": {
           "title": "Quick brown rabbits",
           "body":  "Brown rabbits are commonly seen."
        }
     }
  ]
}</pre>
</div>
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